[mlir][LLVM] Switch `undef` for `poison` for uninitialized values (#125629)

LLVM itself is generally moving away from using `undef` and towards
using `poison`, to the point of having a lint that caches new uses of
`undef` in tests.

In order to not trip the lint on new patterns and to conform to the
evolution of LLVM
- Rename valious ::undef() methods on StructBuilder subclasses to
::poison()
- Audit the uses of UndefOp in the MLIR libraries and replace almost all
of them with PoisonOp

The remaining uses of `undef` are initializing `uninitialized` memrefs,
explicit conversions to undef from SPIR-V, and a few cases in
AMDGPUToROCDL where usage like

    %v = insertelement <M x iN> undef, iN %v, i32 0
    %arg = bitcast <M x iN> %v to i(M * N)

is used to handle "i32" arguments that are are really packed vectors of
smaller types that won't always be fully initialized.
40 files changed
tree: e973f5c6719610fc1fe239c22c518142c6cbbc2d
  1. .ci/
  2. .github/
  3. bolt/
  4. clang/
  5. clang-tools-extra/
  6. cmake/
  7. compiler-rt/
  8. cross-project-tests/
  9. flang/
  10. libc/
  11. libclc/
  12. libcxx/
  13. libcxxabi/
  14. libunwind/
  15. lld/
  16. lldb/
  17. llvm/
  18. llvm-libgcc/
  19. mlir/
  20. offload/
  21. openmp/
  22. polly/
  23. pstl/
  24. runtimes/
  25. third-party/
  26. utils/
  27. .clang-format
  28. .clang-tidy
  29. .git-blame-ignore-revs
  30. .gitattributes
  31. .gitignore
  32. .mailmap
  33. CODE_OF_CONDUCT.md
  34. CONTRIBUTING.md
  35. LICENSE.TXT
  36. pyproject.toml
  37. README.md
  38. SECURITY.md
README.md

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